In producing integrated circuits by photoetching, a glass mask on which an integrated circuit pattern has been formed using a metal, such as chromium, is placed on a photoresist layer coated on a silicon substrate, and is then exposed to light. After the exposure, processings such as development and etching are conducted to obtain the desired integrated circuit.
Such glass masks for use in photoetching are preferably made of glass compositions which meet the following requirements:
(1) They permit sharp patterns to be formed thereon which do not have defects such as pinholes in a chromium film. Further, they have high adhesion to the chromium film and are resistant to a heat treatments or supersonic wave cleaning.
(2) They are resistant to strong acids and/or alkalis which are used in, for example, washing treatments and peeling apart photoresist layers.
(3) They have a relatively small coefficient of thermal expansion and are not subject to dimensional changes with the lapse of time.
(4) They are free of air bubbles, contaminants, cords, and the like.
Conventional glass compositions for photoetching masks include SiO.sub.2 -B.sub.2 O.sub.3 -RO-R.sub.2 O and SiO.sub.2 -Al.sub.2 O.sub.3 -RO glass compositions.
The SiO.sub.2 -B.sub.2 O.sub.3 -RO-R.sub.2 O compositions do not meet requirement (1) above since they contain alkali oxides and B.sub.2 O.sub.3. In particular, they suffers from problems such as defects, e.g., pinholes, are readily caused in the chromium pattern film and the adhesion of the chromium pattern film is inferior. Furthermore, they have the disadvantage that they can be used only in the preparation of integrated circuits having a low degree of integration since they have a relatively high coefficient of thermal expansion, resulting from the presence of alkali oxides.
On the other hand, the SiO.sub.2 -Al.sub.2 O.sub.3 -RO compositions are free from such defects since they do not contain alkali oxides and B.sub.2 O.sub.3. They have, therefore, characteristics that are satisfactory to a certain extent for the preparation not only of photoetching masks for the production of integrated circuits having a low degree of integration, but also for the preparation of masks for the production of integrated circuits having a high degree of integration.
The SiO.sub.2 -Al.sub.2 O.sub.3 -RO composition, however, has the serious disadvantages that it is difficult to remove air bubbles and cords in the production of a glass therefrom since it has a high melting point and exhibits high viscosity at the time of melting; further, the formed glass is readily contaminated with refractory bricks, etc., used in a fusing furnace. The presence of air bubbles and contaminants having a size as small as several microns is a critical defect in a mask glass.
Methods which are known to lower viscosity at the time of melting include: (1) a method in which B.sub.2 O.sub.3 is added in the case of Glass 7059, etc., produced by Corning Glass Corp., which glasses are commercially available; (2) a method in which the ratio of the RO component to the SiO.sub.2 -Al.sub.2 O.sub.3 component is increased; and (3) a method in which ZnO is added as the RO component, as is described in Japanese Patent Publication No. 54252/81.
Method (1) above, however, as has already been described above, may cause the formation of defects such as pinholes in the pattern film, and reduce the adhesive force of the pattern film. Method (2) is not recommendable in that it increases the coefficient of thermal expansion of the formed glass. Method (3) is disadvantageous in that the viscosity-decreasing effect of ZnO is small, it should be used in the form of zinc oxide (ZnO) and cannot be used in the form of ZnCO.sub.3 or Zn(NO.sub.3).sub.2, which have the effect of removing air bubbles, and, therefore, an insufficient effect of removing air bubbles can be expected.